Troubleshooting — Remote Sensing

SPECTRAL DISCONTINUITIES

The small offset steps occur at the VNIR-to-SWIR1 and SWIR1-to-SWIR2 spectrometer transitions at 1000 nm and 1400 nm. These steps are a function of the slightly offset fields-of-view (FOV) of the fiberoptics for each spectrometer, and the fact that the signal-to-noise is lowest at these points, in combination with the following effects:

1. Shiny parts of the target that create significant specular components (see attached technical documents). These effects are mitigated as follows:
   1. Reposition the fiberoptic input, target, and/or the illumination source to avoid specular illuminations and views.
2. Significant differences in target geometry such as with large textures, either angular or distance differences, and the associated illumination differences due to those geometries (see attached technical documents). These effects are mitigated as follows:
   1. Rotate the target until all surfaces receive similar illumination levels;
   2. Change the fiberoptic input position to view a section of the target more evenly illuminated;
   3. If the illumination is a point source, as most are, move the illumination source further away to reduce the distance difference effects;
   4. Move the illumination source to a more direct, straight-on angle to as many of the target surfaces as possible to reduce angular difference effects;
   5. Use a collimated light source;
   6. Use an 8 deg foreoptic (the lens focal point view is a little more 'fuzzy/mixed' at longer distances than the bare fibers); and/or
   7. Make measurements in irradiance using the diffuser type cosine collector.
3. Dark Current DC drift over time as the spectrometer reaches thermal equilibrium, or significant changes in ambient temperature that effect DC . These effects are mitigated as follows:
   1. ASD Drift-Lock, which deals with small DC drift effects by automatically updating DC from: some masked channels in the VNIR array and a fast signal capture of the scanning SWIR detectors as the SWIR shutter swings over;
   2. Allow the spectrometer to reach thermal equilibrium by letting it run for about 20 minutes before re-optimizing and new DC collection;
   3. Take a new DC more frequently (new DC temporarily closes the built-in shutter over the entire VNIR array); and/or
   4. Make measurements in radiance or irradiance and use the P-Correct function.
4. Significant differences in illumination over the same target plane or differences in reflectance within the same plane such as, a laser dot moving across a diffuse panel within the FOV or a flat target that abruptly goes from high to low reflectance within the FOV. These effects are dealt with as follows:
   1. Use the ASD Scrambler; and/or
   2. Average several different views over the target.